Dissection of cytokine receptor-linked mitogenic signaling pathways by complementation with polyomavirus middle T antigen (MT)

The proliferation of activated T cells is triggered by the binding of the principal T cell growth factor, IL-2, to high-affinity IL-2 receptors (IL2R). The IL-2R comprises three subunits, o, , and yc. The /yc heterodimer is both necessary and sufficient for the initiation of signaling events leading to cell-cycle progression and proliferation. The acidic domain of IL-2R has been implicated in the activation of three key signaling proteins, Src-family kinases, phosphatidylinositol 3 kinase (PI3K) and the She adaptor protein, which may couple the receptor to the Ras pathway. Deletion of the acidic domain of the IL-2R (IL-2RAA) renders the receptor non-functional. To examine the the role of each signaling pathway in IL-2R-induced mitogenesis, we have developed a complementation assay based on the ectopic expression of chimeric IL-2R in the myeloid progenitor cell line, FDC-P1. In these cells, stimulation of the chimeric IL-2R/Y, but not the IL-2RAA/YC heterodimer induces proliferation. This defect was complemented by coexpression of MT, a viral oncoprotein known to bind and activate She, PI3K and Src family kinases. In contrast, an MT mutant that fails to bind She was profoundly defective i complementing activity, whereas a second mutant lacking the PI3K binding site exhibited a less dramatic impairment of function in this assay. These results suggest that a signaling pathway involving She plays an important role in mitogenesis. (This work is supported by ACS grant #19).